1. Field of the Invention
The present invention relates to a protecting device of an injection molding machine provided with a hydraulic cylinder having a built-in double-rod type piston and a servo valve for driving and controlling the hydraulic cylinder.
2. Description of the Relevant Art
Up to now, there is known an injection molding machine provided with an injection cylinder having a built-in double-rod type piston and a servo valve for driving and controlling the cylinder, in the official gazette of Japanese Patent Laid-Open No. 5(1993)-50483.
FIG. 6 shows a schematic construction of this kind of injection molding machine. In the same drawing, the injection molding machine 50 is provided with an injection apparatus 51 and a mold clamping apparatus 52. The injection apparatus 51 is provided with a barrel cylinder 54 having a built-in screw 53. The barrel cylinder 54 has an injection nozzle 55 at a fore end thereof and a hopper 56 for supplying a material to be molded, at a rear end thereof. Also, on the rear end of the barrel cylinder 54 is disposed an injection cylinder 57 for driving the screw 53 in a fore and rear direction thereof. The injection cylinder 57 has a built-in double-rod type piston 58, and a rear end of the screw 53 is connected to a fore end of the piston 58. Further, an oil motor 59 is disposed on a rear end of the injection cylinder 57, and a rotational shaft 60 of the oil motor 59 is spline-connected to a rear end of the piston 58. A fore oil chamber and a rear oil chamber of the injection cylinder 57 are connected to a pressurized oil supplying portion 62 and an oil tank 63 through a four-port servo valve 61. Moreover, the pressurized oil supplying portion 62 includes a load/unload changing valve 65 of a hydraulic pump 64, a non-return valve 66 and an accumulator 67. Reference numeral 68 denotes a control circuit for controlling a servo valve 61, and the control circuit 68 is fed with electricity from a power source portion 69.
The injection molding machine 50 is provided with the injection cylinder 57 having the built-in double-rod type piston 58, and also drives and controls the injection cylinder 57 by means of the servo valve 61, so that positional control (stopping control) of the screw 53 is easily carried out.
Sometimes the injection and charging is intended to be completed in a short time in order to hinder the progress of solidification before the completion of the injection and charging. Usually, the accumulator 67 is connected to the side of the pressurized oil supplying portion 62 and the servo valve 61 is made large-sized, so that the injection speed is high.
However, in a case where the positional control is carried out for the screw 53 in the injection molding machine 50 which is made high speed, even if the piston 58 is stopped, hydraulic pressure is always applied to the fore oil chamber and the rear oil chamber in the injection cylinder 57. Accordingly, in a case where a power failure is caused, and then a voltage of an AC power source is decreased to 0 volts at a tp point shown in FIG. 7(A), a power source input voltage E (+E, -E) of the control circuit 68 is not decreased instantaneously as shown in FIG. 7(B), and based on which, an instruction signal Sc given to the servo valve 61 turns out to be in an unstable uncontrolled state in a period Zc shown in FIG. 7(C). This unstable period Zc is a short time. However, in case of a high speed injection molding machine, the injection is completed at about 0.1 second, so there is a problem that the resin is ejected out of the injection nozzle 55 in a little time at a time of the power failure. Moreover, the operation of the screw 53 is interlocked by means of a purging cover in order to prevent the resin from scattering in a normal high speed injection, so that the screw 53 cannot be advanced without closing the purging cover. However, such a protection countermeasure is ignored in the uncontrolled state, and then the resin is ejected in an unguarded state.